1. Field of the Invention
The present invention relates to computing devices that communicate with each other within a wireless local area network, and more particularly, to computing devices that operate in an environment in which both frequency-hopping and direct-sequence spread spectrum radio frequency signals are present and which are capable of discriminating between both types of RF signals.
2. Description of Related Art
A wireless local area network (WLAN) comprises a plurality of remote computing devices which communicate together over radio frequency (RF) signals. As in a wired local area network (LAN), the WLAN allows users to seamlessly access disk drives, printers, and additional computer resources connected to the WLAN. The remote computing devices include a radio receiver/transmitter specifically designed or adapted for RF communication with the other elements of the WLAN. The WLAN may also include a central host processing unit that sends information to and receives information from any one of the plurality of remotely disposed computing devices. The central host processor may also form part of a separate wired LAN to provide a bridge with the WLAN. In such a WLAN, the remote computing devices may comprise portable units that operate within a defined environment to report information back to the central host processing unit. WLAN systems offer increased flexibility over wired LAN systems by enabling operators of the remote computing devices substantial freedom of movement through the environment, and are particularly useful for remote data collection applications such as inventory control, manufacturing and production flow management, and asset tracking.
For simplicity, the radio receiver/transmitter provided within each remote computing device may communicate using conventional narrowband RF signals. Narrowband RF operation has a significant drawback in that the radio receiver/transmitter must be operated at relatively low power levels in order to ensure compliance with certain governmental regulations. In addition, low power level signals are more susceptible to interference and thus have low data throughput rates. To overcome these and other drawbacks, commercial WLAN systems have adopted so-called "spread spectrum" modulation techniques. In a spread spectrum system, the transmitted signal is spread over a frequency band that is significantly wider than the minimum bandwidth required to transmit the information being sent. As a result of the signal spreading, spread spectrum systems enable high data integrity and security. Moreover, by spreading transmission power across a broad bandwidth, power levels at any given frequency within the bandwidth are significantly reduced, thereby reducing interference to other radio devices.
In one type of spread spectrum communication system, a modulated RF carrier is moved in discrete increments in a pattern dictated by a pseudorandom sequence. This type of spread spectrum system is known as a "frequency-hopping" modulation system, since the transmitter jumps from frequency to frequency in accordance with the pseudorandom sequence. The information signal is modulated onto the carrier using frequency shift keying (FSK) or other known types of modulation. The instantaneous frequency-hopping spread spectrum signals are similar to conventional narrowband RF communications, except that the center frequency of the signals moves in the pseudorandom sequence with the carrier impressed upon the center frequency.
A second type of spread spectrum communication system utilizes an RF carrier modulated by a digital code sequence having a spreading code rate, or chipping rate, much higher than the clock rate of the information signal. This type of spread spectrum system is known as a "direct-sequence" modulation system, and the modulated signals have a much wider bandwidth than narrowband RF signals or frequency-hopping signals for an equivalent data rate. The RF carrier may be modulated such that a data stream has one phase when a spreading code sequence represents a data "one" and 180.degree. phase shift when the spreading code sequence represents a data "zero." The RF carrier may also be binary or quadrature modulated by one or more data streams such that the data streams have one phase when a spreading code sequence represents a data "one" and a predetermined phase shift (e.g., 180.degree. for binary, and 90.degree. for quadrature) when the spreading code sequence represents a data "zero." These types of modulation are commonly referred to as binary shift key (BPSK) and quadrature shift key (QPSK) modulation, respectively.
It is sometimes desirable to operate remote computing devices in an environment in which both direct-sequence (i.e., wideband) and frequency-hopping (i.e., narrowband) RF communications are present simultaneously. For example, a single WLAN may include disparate elements operating in each of the two modes while sharing a single common host processing unit. In such an environment, it is desirable for the receiver circuitry within the remote computing devices to differentiate between the two types of signals to avoid performance impacts. If an interfering narrowband signal is erroneously interpreted by the receiver as a valid information signal, the receiver could hold off other pending transmissions while trying to synchronize to the narrowband signal, resulting in reduced system throughput and degraded performance.
Thus, it would be desirable to provide a receiver which can discriminate between narrowband and wideband RF signals for use in an environment in which both types of signals are present.